Inband signalling system



Oct. 30, 1962 w. E. NoLLER INBAND SIGNALLING SYSTEM FiledApril so, 195eIN V EN TOR. WAL TER E. /I/OL El? BY f Za/Z T TORNEI/S ilnited tates3,061,783 INBAND SIGNALLlNG SYSTEM Walter E. Noiler, Berkeley, Calif.,assignor to Lynch Carrier Systems, lne., a corporation Filed Apr. 30,1956, Ser. No. 581,438 6 Claims. (Cl. 32E-64) My invention relates tothe art of communication and is especially concerned with anelectro-magnetic system in which intelligence substantially at voicefrequency is transmitted from a sending station to a receiving station.In such a system, it is customary to have at the receiving station notonly equipment which is designed to render audible the received voicefrequencies but also other equipment actuated by means of variousenergizing impulses. This auxiliary equipment may be dialing or ringingmechanism, for example. It is customary to provide the energizingimpulses at some frequency other than the voice frequencies normallytransmitted and to have such impulses remotely spaced in the frequencyspectrum from the voice frequencies.

If the auxiliary equipment were responsive to voice frequencies, thennormal conversation over the communications circuit would from time totime and in unwanted instances actuate the auxiliary equipment. To avoidunwanted or inadvertent, random actuation of auxiliary equipment it hasbeen necessary heretofore to resort to various means involving departurefrom the voice spectrum for handling the impulses for uniquelyenergizing the auxiliary equipment. There is an increasing necessity toutilize as narrow a frequency spectrum as possible for directcommunication and also for auxiliary equipment, and so it is highlydesirable to reduce the frequency spectrum required for all purposes.

It is therefore an object of my invention to provide a signalling systemfor use with voice frequency intelligence transmission in which theimpulses for actuating auxiliary equipment are within the voicefrequency spectrum, yet in which the voice itself does not inadvertentlyor at random actuate lthe auxiliary equipment.

Another object of the invention is to provide an inband signallingsystem in which impulses for actuating auxiliary equipment aretransmitted eectively at frequencies Within the voice spectrum yet inwhich the voice itself does not produce unwanted actuation `of theauxiliary equipment.

Another object of the invention is to -provide a simple andstraightforward means for distinguishing, within a voice frequencyspectrum, signals for actuating auxiliary equipment.

Another object of the invention is to provide auxiliary equipmentresponsive to signals Ywithin the voice frequency spectrum but notsubstantially actuated by the voice.

Another object of the invention is, 4in general, to improvecommunication signalling systems.

A still further object of the invention is to provide a practical inbandsignalling system of a relatively simple, effective nature.

An additional object of the invention is to provide an inband signallingsystem not dependent upon complex guard circuits, band elimination andelaborate filter combinations.

Other objects, together with the foregoing, are attained in theembodiment of the invention ldescribed in the accompanying descriptionand illustrated in the accompanying drawings, in which- FIGURE l is ablock diagram of an inband signalling system arranged in accordance withlthe invention.

FIGURE 2 is a diagram showing the relationship of various signal plotsin the inband signalling system.

While the inband signalling system of the invention 3,661,783 PatentedOct. 30, 1962 ice can be utilized in connection with various means oftransmitting intelligence and can be adapted directly or by analogue towidely variant forms of such means, i-t has successfully beencommercially embodied substantially as illustrated herein. In thisarrangement, there is provided a voice sending station 6 of anyrecognized sort in which the energy of the human voice at its cus-tomaryspeech frequencies is translated into electro-magnetic impulsesimpressed on a conductor 7 and transmitted over a transmission path 8.This path may be with or without metallic conductors and is especiallydesigned for the efficient and effective carriage of the intelligenceimpressed at the sending station 6. One example of the transmission path8 is a carrier telephone circuit. The path 8 extends to a conductor 9ending at a voice receiving station 'lil at which -the electro-magneticenergy is translated into a reasonabel reproduction of the soundimpressed at the sending station 6. The frequency spectrum of thereproduced energy at the receiving station 10 is approximately the sameas the frequency spectrum of the energy impressed upon the sendingstation 6.

Pursuant to the invention, there is provided in connection with thesending Istation 6 and usually, although not necessarily at the physicallocation of the sending station 6 or adjacent thereto, a means (enclosedby the broken line 11) comprising elements eifective to send a specialinband signal over the transmission path 8. While the special signal iswell within the frequency spectrum of the voice sending station 6, italso has certain characteristics largely distinguishing it from thevoice. For that reason, included in the signal sending means il isvapparatus for amplitude modulating two alternately occurring carriers ofdifferent frequency. Part of the apparatus is a primary oscilaltor 12.This oscilaltor is of any convenient kind and has no specialcharacteristics except that it is operable orituna'ble to operate ateither one of two selected frequencies. The first selected orpredetermined frequency is well within the voice spectrum and the secondpredetermined or selected frequency, although of a different value andseparated from the first frequency by a signicant amount, is itselfstill within the frequency spectrum of the voice. l

The primary oscillator 12 is arranged to be operated at one of theselected frequencies for a given interval of time and then at the otherof the selected frequencies for a given interval of time so that thefrequencies'follow or alternate with each other at given orpredetermined times. It is possible to have more than two predeterminedfrequencies of operation of the oscillator 12 and three or more suchfrequencies are referred to as a sequence or series of such frequencies.Inthe event there are more than two, then the different frequencies arearranged to follow each other at intervals of time and in a sequencewhich is predetermined. The term alternating is there'- fore intended toapply not only to two frequencies which succeed each other but also to aplurality of distinct frequencies which follow each other in selectedorder and for chosen individual duration times.

The oscillator 12 can be switched between the two (or more) frequenciesby any convenient means including me'- chanical or other switching and,in fact, can be substituted for by a pair (not illustrated) ofindividual oscillators each of which has one of the predeterminedfrequencies of operation, with the switching being done between them.Preferably, the primary oscillator 12 (or the primary oscillators) hasthe function of giving out energy at different frequencies Within thevoice spectrum under the control of a secondary oscillator 13. Thefrequency of oscillation of the oscillator 13 is likewise Well withinthe voice spectrum. The secondary oscillator 1'3 is utilized as aswitching oscillator. This can conveniently be done by connecting theoutput of the secondary oscillator 13 to switch the output of theprimary oscillator 12 so that the two (or more) selected frequencies ofthe primary oscillator 12 alternate at the selected switching frequencyof the secondary oscillator 13. l

This condition is represented in FIGURE 2 by the sine sequence betweenthe lines 14 and 16 showing an oscillation plot 17 of a set amplitudeand modulated at a selected frequency designed f1 and having a timeduration t1 and by the sinusoidal oscillation plot 18 between the lines1'6 and 19, The plot 1S is at the set amplitude of the first oscillationf1 but is modulated at a significantly greater frequency represented inthe figure as f2 and extending over a time interval t2 which, althoughnot necessarily so is illustrated as equal in length to the timeinterval t1. As indicated by the plots between the lines 14, 16, 19, 20and 21, the various oscillations at different frequencies succeed eachother repeatedly. i

Also as illustrated in FIGURE 2, the alternations between the primaryoscillator frequencies f1 and f2 can be represented by a sinusoidal plot22 of fixed amplitude and frequency extending either side of its axis23. When on the positive side of the axis, the plot 22 representsproduction of the frequency f1 during the time interval t1 and when onthe negative side of the axis 22, the plot 22 represents the productionof the frequency f2 for an equal time interval t2. The undulations ofthe plot 22 represent the frequency of alternation impressed by thesecondary oscillator 13 upon the primary oscillator 12 so that the curve22 represents the frequency f3. The points such as 24 and 25 and 26 atwhichV the plot 22 crosses the axis 23 are spaced to represent the timesat which the frequency changes.

The curve 22 representing the frequency f3 need not be sinusoidal butcan be of lany shape so long as it alternates on opposite sides of theaxis 23 and crosses the axis at the points 24, 25 and 26 at the times offrequency switch. Thus, in any case, FIGURE 2 represents the operationof the secondary oscillator 13 and of the primary oscillator 12 togenerate continuously a signal made up of voice frequency modulationsand comprised of at least two primary oscillations or two tones each ata voice frequency different from the other and themselves alternated ata set frequency; that is, modulated at the voice frequency or third toneof the'secondary oscillator. The term tri-tone is conveniently appliedto the system.

While the production of the three tones or compound voice frequencysignal from the primary and secondary oscillators may be continuous, itis not impressed continously upon the transmission path 8. Rather, thissig- `nal as generated is itself governed by an amplitude modulator 27.This modulator can be of any convenient kind effective to vary theamplitude with which the compound oscillator signal is impressed uponthe transmission path V8. For example, the modulator 27 may include aswitch 28 which opens the conductor interposed-between the oscillators12 and 13 and the transmission path 8.

The switch 28 can be opened and closed so that when the path iscontinuous the impulse from the oscillators 12 and 13 travels over thetransmission path `8 and when the switch 28 is open there is notransmission of the signal impulse over the transmission path 8. Theclosure of the switch 28 is represented by the amplitude modulated curve29 in FIGURE 2.

v i switch 28 is closed the three tone signal energy from the signalmeans 11.

Although not necessarily so located, there is conveniently provided ator adjacent the receiving station 10 an impulse receiver, generallydesignated '31, which includes certain auxiliary equipment 32 designedto be appropriately actuated in connection with the transmission of theintelligence. In a telephone installation, for example, the mechanism 32might be a ringing device. The receiving means 31 is connected to thetransmission path 8 and preferably includes a signal limiter 33. Thelimiter in effect reduces amplitude variation of frequencies from voicetransmission along the path 8 to a very low value, too low to have anactuating effect upon the auxiliary equipment 32. The relatively strongsignal from the modulator 27, being substantially deprived of amplitudevartion by the limiter, now appears at the output of 33 substantially asthe wave form 17, 18 of FIG. 2. It is passed into a discriminator 34wherein the frequency f3 of the secondary oscillator is recovered, andappears at the output thereof substantially in the form shown at 22 inFIG. 2.

The output 22I of the discriminator 34 is fed into a band pass lter 36designed to block all frequencies other than the frequency f3 and passonly the frequency f3, the frequency at which the secondary oscillator13 is effective. Sincethe other frequencies are screened out, only thefrequency f3 is passed to a rectifier 37 in which the signal originallyimpressed by the amplitude modulator 27 is detected and recreated tosubstantially the form shown at 29, and so serves as an impulse foractuating the auxiliary device 32. Thus, the three tone signal which isput upon the transmission path S at the output of the amplitudemodulator 27 is ultimately effective to actuate the auxiliary equipment32.

Since all of the frequencies utilized in the signalling means 11 andtravelling over the transmission path 8 are within the voice frequencyspectrum, it might be thought that voice frequencies impressed upon thetransmission path at the sending station l6 might themselves travelthrough the receiving means 31 and be eective to actuate the auxiliaryequipment 32. That, however, is not the case in fact, certainly not in acommercial sense. It has been determined by careful study of voiceyfrequency patterns and combinations and by extensive laboratory andcommercial experience that the frequencies f1 and f2 of the primaryoscillator 12 when alternated or utilized in sequence at the frequencyf3 of the oscillator 13 do not occur in voice transmission.

Perhaps it can not be said that such a combination of voice frequenciescan never occur in speech, still in laboratory tests and in commercialoperation over a protracted period of time, no actuation of auxiliaryequipment by such a combination has ever occurred. It certainly can besaid that the incidence or production in the voice of the frequencies f1and f2 at the modulation frequency f3 at all, or for the amplitudemodulation interval 29, occurs so seldom in fact and in commercial usageas to be well Within the failure expectancy of other items of equipmentutilized in the system. For that reason, in the claims herein there isreference to the substantial non-occurrence within voice transmission ofthe frequencies 11 and f2 modulated at the frequency f3. While it cannotbe said that it is impossible for the auxiliary equipment 32 to beactuated by the voice sending equipment 6, yet when the auxiliaryequipment 32 is set up with the translating equipment 33, 34, 36 and 37,the statistical probability of any unwanted actuation occurring is soremote that it can be said substantially never to occur. The probabilityof occurrence of an unwanted actuation is even less than the probabilityof other `failures which occasionally occur in a practical, commercialsystem.

There has thus been provided, pursuant to the invention, an inbandsignalling system utilizing two base frequencies or tones alternated ormodulated at a third frequency or tone to provide a compound signalamplitude modulated for transmission at voice frequency over a voicefrequency transmission path and which at the receiving station is sointerpreted that it actuates auxiliary equipment and in which theinterpretation equipment substantially never receives from the voicetransmission a signal which duplicates the compound signal. Thus, thereis provided a practical inband signalling system for a voicetransmission path so that auxiliary equipment can be accurately andpositively actuated substantially without false actuation and withoutexpanding the necessary frequency spectrum beyond that of the voice. Ina commercial installation, it is feasible to provide a differential ofto db between unwanted voice and the desired signal and the performanceof the system is not materially changed by variation in magnitude of thesignal tones from O to 20 db below speech.

What is claimed is:

1. An inband signalling system comprising means for receiving over avoice frequency transmitting path a pulse substantially differing fromvoice frequencies and including an amplitude modulated signal resulting`from the transmissi-on of two different voice frequencies alternatingat a third voice frequency, said means including in sequence a limiterfor substantially limiting amplitude variations of said signal, adiscriminator for recovering said third frequency from said signal, aband pass filter for passing substantially only said third frequency,and a detector for recovering said amplitude modulated signal.

2. An inband signalling system comprising a sending station, a receivingstation, means for transmitting intelligence at voice frequenciesbetween said stations, means located at said sending station forimpressing upon said transmitting means an amplitude modulated signalincluding a pair of different frequencies alternating at a thirdfrequency, means located at said receiving station for limiting theamplitude of said signal, means located at said receiving station fordiscriminating said third frequency in said signal, means located atsaid receiving station for filtering out all but said third frequencyfrom said signal, and means located at said receiving station fordetecting the amplitude modulation of said third frequency remaining insaid signal.

3. An inband signalling system comprising a sending station, a receivingstation, means for transmitting intelligence at voice frequenciesbetween said stations, means located at said sending station forimpressing upon said transmitting means a signal resulting from theamplitude modulation of two different frequencies alternating at a thirdfrequency, means located at said receiving station for limiting voiceamplitude modulations of said signal, means located at said receivingstation for discriminating to recover said third frequency of saidsignal, means located at said receiving station for passingsubstantially only said third frequency, and means located at saidreceiving station for detecting said amplitude modulation of said thirdfrequency remaining in said signal.

4. System for transmitting a control signal over an audio frequencytransmission system and employing audio frequencies to effect saidcontrol, comprising: a transmission system, audio means for applying tosaid transmission system transmission signals covering a range of audiofrequencies, means for producing a control signal residing in the audiofrequency range, means for frequency modulating said control signal at apredetermined modulating frequency also residing in the audio frequencyrange, means for applying said frequency modulated control signal tosaid transmission system, means for selectively amplitude modulating thecontrol signal applied to said transmission system, means for receivingfrom said transmission system the transmission signals applied to saidtransmission system by said audio means,

means for receiving from said transmission system said control signal,demodulating means for frequency demodulating said received controlsignal to substantially recreate said predetermined modulatingfrequency, filter means for receiving the output of said demodulatingmeans and for passing only a narrow band of frequencies centeredsubstantially at said predetermined frequency, and a respondingmechanism effective to respond to signal passed by said filter meansduring the sustaining of said frequency modulated control signal,thereby to respond and thus recreate substantially the selectiveamplitude modulation applied to said control signal.

5. System for transmitting a control signal over an audio frequencytransmission system and employing audio frequencies to effect saidcontrol, comprising: a transmission system, audio means for applying tosaid transmission system transmission signals covering a range of audiofrequencies, means for producing a control signal residing in the audiofrequency range, means for frequency modulating said control signal at apredetermined modulating frequency also residing in the audio frequencyrange, means for applying said frequency modulated control signal tosaid transmission system, means for selectively amplitude modulating thecontrol signal appiied to said transmission system, means for receivingfrom said transmission system the transmission signals applied to saidtransmission system by said audio means, means for receiving from saidtransmission system said control signal, means for frequencydemodulating said received control signal to substantially recreate saidpredetermined modulating frequency, and a responding mechanismselectively responsive only to a narrow band of frequencies centeredsubstantially at said predetermined `frequency and effective to respondduring the sustaining of said frequency modulated control signal,thereby to respond and thus create substantially the selective amplitudemodulation applied to said control signal.

6. Inband signalling system for transmitting and receiving audiofrequency signal and a control signal, all significant frequencies ofsaid signals lying in the audio range, comprising: a transmissionsystem, means for applying to said transmission system audio frequencysignals covering a range of audio frequencies, means for applying tosaid transmission system a control signal, means for successivelyalternating the carrier frequency of said control signal between twodifferent frequencies at a third predetermined alternation frequency,means for selectively applying said control signal to said transmissionsystem, means for receiving said audio signals from said transmissionsystem, means selectively responsive to the carrier frequencies of saidcontrol signal for receiving said control signal from said transmissionsystem, means for demodulating the output of said last name receivingmeans to create a signal of said predetermined frequency, filter meansselectively responsive to a narrow band of frequencies centeredsubstantially at said predetermined frequency for delivering an outputin response to application thereto of said narrow band of frequencies,and means responsive to the output of said filter means for respondingto said control signal whenever the same is applied to said transmissionsystem by said applying means.

References Cited in the file of this patent UNITED STATES PATENTS2,299,821 Horton Oct. 27, 1942 2,461,956 Beckwith Feb. 15, 19492,495,452 Grove Ian. 24, 1950 2,535,104 Van Mierlo Dec. 26, 19502,547,024 Noble Apr. 3, 1951 2,871,463 Beckwith Ian. 27, 1959

